1. Field of the Invention
This invention relates to a motor unit used in the rotary portion of a precision rotation instrument such as a magnetic disk or Video tape Recorder.
2. Description of the Prior Art
The heretofore known motor unit of this type is of a structure in which, as disclosed, for example, in Japanese Laid-open Patent Application No. 200722/1982, a rotatable member is supported by two ball bearings. In such a support structure using ball bearings, it is necessary to use the motor unit with a preload applied to the bearings by preloading means such as predetermined position preloading means and constant preloading means.
However, the preloading means is structurally complicated and therefore requires much time and labor in assembly, thus resulting in high cost. Also, the bearing is of a structure in which outer and inner races are formed as individual members, and this has led to a disadvantage in that if a clearance fit is provided between the shaft and the inner race, rotation accuracy is reduced. Conversely, if an interference fit is provided between the shaft and the inner race, assembly becomes more difficult.
It is also known to use a hydrodynamic pressure type fluid bearing instead of said ball bearing (see Japanese Laid-open Patent Application No. 97920/1982), but where a fluid bearing is used, it is necessary to provide a thrust bearing at the end of the shaft and, the structure becomes complicated.